Section 5. When clerks cause copies to be printed which they are required to furnish, they shall make no charge for such printed copies in excess of the amount actually paid for the printing thereof. They may require the estimated cost of said printing to be paid in advance, and they shall supervise the printing and correct the proofs without charge. All written copies, including such as are prepared for printing, shall be charged for at the rate of twenty cents a page; provided, that clerks shall, upon the request of the chief police officer of any city or town, or of any district attorney within the commonwealth, furnish without charge to such chief police officer or district attorney certified copies of criminal records in their custody.